In modern communication systems, high-speed digital signals are typically passed through transmission channels and/or media that are less than ideal. The transmission channel and/or media transmission characteristics may degrade a transmitted original digital signal to the point that a receiver is unable to accurately differentiate between a received zero and/or one in the received digital signal at the receiver. This problem is more acute for communication test systems that are utilized to test and characterize numerous types of electronic devices (generally known as “devices under test” or “DUTs”) because on the need to accurately characterize the DUTs.
One approach to solve this problem includes compensating the deterministic effects introduced by sources such as frequency dependent losses and non-linear phase of the transmission medium, discontinuities from vias and connectors, periodic jitter, duty cycle distortion, etc., to correct the received digital signals using equalization so that the receiver may correctly receive the received digital signals. As an example of this approach, in FIG. 1, a block diagram of an example of an implementation of a known test system 100 is shown. The test system 100 may include a data source 102, transmission channel (i.e., the channel) 104, equalizer 106, and receiver 108. As an example of operation, the data source 102 may send a digital input signal 110 through the channel 104 to the equalizer 106. It is appreciated by those skilled in the art that the channel 104 is typically less than ideal and therefore usually degrades the digital input signal 110 based on the transmission characteristics of the channel 104. As a result, the channel output signal 112 is the digital input signal 110 degraded by the transmission characteristics of the channel 104. The equalizer 106 then receives the channel output signal 112 and equalizes the channel output signal 112 in an attempt to compensate for the transmission characteristics of the channel 104. The resulting equalized output signal 114 is then passed to the receiver 108.
Examples of the channel 104 in a typical test system 100 are shown in FIGS. 2 and 3. In FIG. 2, a block diagram of an example of an implementation of a known channel 200 in the test system of FIG. 1 is shown. In this example, the channel 200 may include an input cable 202 and an output cable 204. In FIG. 3, a block diagram of another example of an implementation of a known channel 300 in a test system is shown. In this second example, the channel 200 may include the input cable 202 and output cable 206 shown in FIG. 2 and a DUT 302. It is appreciated that by utilizing both implementations that the test system may be calibrated so as to measure the transmission characteristics of the DUT 302.
An example of a known equalizer 106 is shown in FIG. 4. A common type of equalizer is the linear feed-forward equalizer (“LFE”). The LFE is a finite impulse response (“FIR”) linear filter. In FIG. 4, a block diagram of an example of an implementation of a known LFE 400 is shown. The LFE 400 may include a plurality of n time delays τ of equal length, an accumulator 402, a plurality of n tap coefficients K 404, and a low-pass filter (“LPF”) 406. In an example of operation, the LFE 400 passes an input signal 408 through to both a tap coefficient K0 410 of the plurality of n tap coefficients K 404, via signal path 412, and the plurality of time delays 402 via signal path 414. The tap coefficient K0 410 is multiplied with the input signal 408 and the result is passed to the accumulator 402. Similarly, as the input signal 408 is passed through the plurality of time delays 402, the input signal 408 is time delayed by each time delay in the plurality of time delays 402 the resulting time delayed signals are multiplied with a corresponding tap coefficient (i.e., K1, K2, . . . , Kn) of the plurality of n tap coefficients K 404. The corresponding results are then sent to the accumulator 402 that accumulates the results. The accumulated result 416 is the passed to the low-pass filter 406 which filters the accumulated result 416 and produces the equalized output 418.
Unfortunately, the typical design and evaluation of a high-speed digital transmission network with one or more LFEs 400 involves the derivation of the plurality of n tap coefficients K 404. It is appreciated by those skilled in the art that this usually requires a difficult formal derivation approach with technical expertise utilizing trial and error, inverse filter estimation from S-parameter or TDT channel characterization, or the iterative convergence algorithms of adaptive filters. Therefore, there is a need for a closed form method to determine the n tap coefficient K 404 values. Additionally, there is a need for a system capable of compensating for the deterministic effects of a channel and data source utilizing an LFE.